Control system for power converter

Abstract: A control system is disclosed for a power converter (3) having a plurality of self-turn-off devices and connected to a supply system (1) through a transformer (2) provided with a core. The control system includes circuit for generating a target value for an output voltage of the power converter, and a correction circuit for correcting the target value, based on a value representing magnetic flux of the core of the transformer, to generate a corrected target value. The power converter (3) is controlled to output the target value. DC magnetization of the transformer is suppressed even for the case when DC converter is generated by the supply system. The value representing magnetic flux is obtained either from the primary and secondary currents (i₁, i₂) or from direct flux measurement (16).

Description: This invention relates to a control system for a power converter, and more particularly relates to a control system for a power converter which is composed of self-turn-off devices such as, gate turn-off thyristors (hereafter, simply GTOs), and is connected to a power system or loads via transformers, which can prevent DC magnetization of the transformer. Figure 7 shows a schematic diagram of a prior art control system for a power converter which is composed of GTOs (hereafter, called a self-commutated converter), and is used in a DC transmission system or a static var compensator and so on. In Figure 7: 1 is a power system; 2 is a transformer for connecting a self-commutated converter 3 composed of GTOs and power system 1; 4 is a DC power source such as capacitors etc.; 5 is a current detector which measures the output current of self-commutated converter 3; 6 is a potential transformer which measures the voltage of power system 1; 7 is a control circuit which controls the system voltage according to a system voltage reference 51; 8 is a DC component detector which detects the DC component contained in the output current of self-commutated converter 3 measured by current detector 5; 9 is an adder which adds the output of control circuit 7 which is an instruction value for the output voltage of self-commutated converter 3, and the output of DC component detector 8; 10 is a pulse-width modulation (PWM) control circuit which adjusts the output voltage of self-commutated converter 3 by determining the firing timing of the GTOs in response to the output of adder 9; and 11 is a gate pulse amplifier circuit for generating gate pulses for GTOs in self-commutated converter 3. In Figure 7, PWM control circuit 10 determines the GTO firing pattern so that no DC component is contained in the output voltage of self-commutated converter 3. However, the actual output voltage takes a waveform which contains a DC component due to the variations in the characteristic of the GTO and the variations in the gate signal transmission time. When the output voltage of self-commutated converter 3 contains a DC component, the core of transformer 2 is magnetized asymmetrically because the voltage time product per cycle applied to transformer 2 does not become "0". Thus, the excitation current increases, and the output current of self-commutated converter 3 becomes over-current. This leads the stopping of operation of self-commutated converter 3 for its protection. In the worst case, this sometimes leads to damage to the devices forming the self-commutated converter 3. In the prior art circuit shown in Figure 7, to prevent DC magnetization the following control is performed. That is, the output current of self-commutated converter 3 is detected by current detector 5 and the DC component generated in the course of DC magnetization is detected by DC component detector 8. PWM control is then executed by adding the detected DC component and the instruction value from control circuit 7. Therefore, the output voltage of self-commutated converter 3 is adjusted so that DC magnetization is eliminated. The voltage of power system 1 is usually AC voltage. However, transient DC components may be included in the voltage of power system 1 such as when connecting the power capacitor or the transformer to power system 1. In prior art technology, while it is possible to correct the DC component outputted from self-commutated converter 3, the DC magnetization due to the DC component generated by power system 1 cannot be suppressed, because no correction due to the DC component of the power system 1 side is made. Therefore, when a transient DC component is generated in the power system 1, transformer 2 will be magnetized asymmetrically and lead to an over-current. DE-A-4013171 discloses a device for removing DC components from the output of a multiphase DC/AC convertor comprising a DC source, a switching array, a filter and a transformer. The voltage of each phase on the secondary side of the transformer is detected and compared with a reference signal to generate signals to control the switching of the switching array. The switching signals of two of the three phases are also adjusted in dependence on the output from a respective DC component detector connected between that phase and the third phase. EP-A-0361389 discloses a further device for removing DC components from the output of a multiphase DC/AC converter, comprising a DC source, a DC/AC inverter and a transformer. The signal input to the primary side of the transformer is detected, and a DC component thereof is detected, and the DC/AC inverter . operation is controlled in accordance with the detected DC component. According to the present invention, there is provided a control system for a power converter having a plurality of switching devices and connected to an outer system through a transformer provided with a core, the control system comprising: output voltage instruction value generating means for generating an instruction value for an output voltage of said power converter; correction means for correcting said instruction value to generate a corrected instruction value for said output voltage of said power converter; and signal generating means for generating a signal based on said corrected instruction value, said signal being applied to said switching devices of said power converter to control said output voltage of said power converter, characterised in that: said correction means generates a corrected instruction value in dependence upon a value representing the DC component of the magnetic flux of the core of the transformer. The outer system may comprise a power supply or a load, and the system according to the invention enables suppression of DC magnetization of the transformer even for the DC component generated by the power system side (the outer system). A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: Figure 1 is a schematic block diagram showing a control system for a power converter according to an embodiment of this invention; Figure 1A is a schematic block diagram showing a control system for a power converter according to another embodiment of this invention; Figure 1B is a schematic block diagram showing a control system for a power converter according to a further embodiment of this invention; Figure 2 is a simplified equivalent circuit of the transformer 2 shown in Figure 1; Figure 3 is a schematic block diagram showing a control system for a power converter according to another embodiment of this invention; Figure 4 is a diagram showing the relationship between magnetic flux and excitation current when an irregular AC voltage is applied; Figure 5 is a schematic block diagram showing a control system for a power converter according to a further embodiment of this invention; Figure 6 is a diagram showing the relationship between the excitation current and the magnetic flux in the transformer core; and Figure 7 is a schematic block diagram of showing a prior art control system for a power converter. Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, the embodiments of this invention will be described below. Figure 1 is a schematic diagram of a control system for a power converter according to an embodiment of this invention. In Figure 1, 5B is a current detector which detects the current on the primary winding side of transformer 2. 12 is a subtractor for finding the difference between the transformer secondary winding current detected by a current detector 5A and the transformer primary winding current detected by current detector 5B. Figure 2 shows a general equivalent circuit for transformer 2. In Figure 2, X1 and X2 are leakage inductances of transformer 2; R1 and R2 are copper losses of transformer 2; X0 is an excitation inductance of transformer 2; and i1, i2 and i0 are respectively, a primary current, a secondary current and an excitation current flowing in transformer 2. As can be seen from the equivalent circuit in Figure 2, if, for simplicity, the winding ratio of transformer 2 is taken as 1, excitation current i0 of transformer 2 is equal to the difference between primary current i1 and secondary current i2. Excitation current i0 almost corresponds to the magnetic flux of the core of transformer 2. Therefore, if a DC component included in excitation current i0 is detected, the value of DC magnetization can be estimated. Accordingly, in the embodiment shown in Figure 1, the difference between the primary winding current and the secondary winding current of transformer 2 is computed by subtractor 12. A DC component in the current difference is detected by DC component detector 8. Then the output voltage instruction value from control circuit 7 is corrected by the DC component thus detected so that the DC magnetization is eliminated. As a result, the DC magnetization of transformer 2 can be suppressed. Also, since the correction is made by the excitation current, both DC magnetization due to the DC component included in the output of self-commutated converter 3 and the DC magnetization due to the transient DC component included in power system 1 can be corrected. The remainder of the circuit shown in Figure 1 functions in the same way as the circuit used in the prior art, shown in Figure 7, and explained hereinbefore. Figure 1 shows the embodiment wherein the winding ratio of transformer 2 is taken as 1. This invention is not limited to this embodiment. Figure 1A shows a control system for a power converter according to another embodiment of this invention wherein the winding ratio of transformer 2 is 1 : n. In Figure 1A, 17 is a multiplier which multiplies the secondary winding current of transformer 2 detected by current detector 5A by n. The output of multiplier 17 is applied to subtractor 12, which finds the difference between the transformer primary current and the output of multiplier 17. The following control is executed the same as in the embodiment shown in Figure 1. In case that the winding ratio of transformer 2 is 1 : n, the magnitude of the secondary winding current of transformer 2 is 1/n times that of the primary winding current of transformer 2. In order to calculate the excitation current based on the primary and secondary winding currents of transformer 2, it is necessary to multiply the secondary winding current by n. Multiplier 17 is provided for this purpose. Instead of providing multiplier 17 in Figure 1A, a divider may be provided between current detector 5B and subtractor 12 which divides the primary winding current of transformer 2 detected by current detector 5B by n. Figure 1B shows a control system for a power converter according to a further embodiment of this invention wherein power converter 3 is a three-phase power converter. In this case, transformer 2 is composed of a star-delta connection. In Figure 1B, 5A1, 5A2 and 5A3 are current detectors for detecting the currents on the secondary windings of transformer 2, respectively. 5B1, 5B2 and 5B3 are current detectors for detecting the currents on the primary windings of transformer 2, respectively. As the type of the connection of the primary winding differs from that of the secondary winding in transformer 2, the phase of the primary winding current is different from that of the secondary winding current, so that it is necessary to convert the phase of the detected current. A star-delta conversion circuit 18 is provided for this purpose. The primary winding currents detected by current transformers 5B1, 5B2 and 5B3 are applied to star-delta conversion circuit 18. 121, 122 and 123 are subtractors for finding the differences between the three outputs of star-delta conversion circuit 18 and the secondary winding currents detected by current transformers 5A1, 5A2 and 5A3, respectively. DC component detector 8 detects the DC components contained in the outputs of subtractors 121, 122 and 123 and applies them to adders 91, 92 and 93, respectively. Adders 91, 92 and 93 add the three phase components of the output of control circuit 7 and the three outputs of DC component detector 8, respectively and apply the respective sums to PWM control circuit 10. Instead of providing star-delta conversion circuit 18 in the primary side as shown in Figure 1B, a delta-star conversion circuit may be provided between current transformers 5A1, 5A2 and 5A3 in the secondary side and subtractors 121, 122 and 123. In these embodiments, circuit elements, such as multiplier 17 and star-delta conversion circuit 18, are well known to those skilled in the art. Accordingly, the detailed description of the circuit construction of these circuit elements may be omitted. As described above, according to these embodiments, in a power converter composed of self-turn-off devices and connected to an outer system, such as a power system or a load, via a transformer, both the DC magnetization due to the DC components included in the output of the power converter and the DC magnetization due to the transient DC component included in the outer system can be corrected by: finding the difference between the primary winding current and the secondary winding current of the transformer; detecting the DC component included in this difference; and correcting the output voltage instruction value of the power converter based on this DC component as one of the magnetic flux relating values of transformer 2. Furthermore, the case is considered in which a current including DC component is flowing from power system 1 to self-commutated converter 3. In this case, the DC component of the primary winding current detected by current detector 5B is equal to that of the secondary winding current detected by current detector 5A in Figure 1, and no DC magnetization of transformer 2 occurs. Accordingly, the output of DC component detector 8 is zero, so that no correction to the instruction value outputted from control circuit 7 is executed. But in the prior art control system shown in Figure 7, only the secondary winding current of transformer 2 is detected by current transformer 5. As the DC component is included in the detected current, correction is erroneously made to the instruction value from control circuit 7 by the DC component from DC component detector 8, even though the DC component of the excitation current is zero and DC magnetization of transformer 2 does not occur. In such a case, this embodiment has the merit that an erroneous correction can be avoided. The following is a description of another embodiment of this invention with reference to the schematic diagram in Figure 3. In Figure 3, 19 is a phase detection circuit, such as a phase locked loop circuit, which outputs a signal at every fundamental period of power system 1. This signal is applied to a maximum value detection circuit 13 and a minimum value detection circuit 14. The current difference between the transformer primary current and secondary current computed by subtractor 12 is also applied to maximum value detection circuit 13 and minimum value detection circuit 14. Maximum value detection circuit 13 and minimum value detection circuit 14 detect the maximum and the minimum values of the current difference at every fundamental period of power system 1, respectively. 15 is an arithmetic circuit which computes the center value of the current difference at every fundamental period of power system 1 from the maximum and minimum values. The center value is applied to adder 9. When this embodiment is composed in this way, as shown in Figure 4 (a), even when a period occurs in which constant values continue for both magnetic flux and excitation current due to the application of an irregular AC voltage, the DC magnetization can be more accurately grasped by detecting the center value of the magnetic flux as that representing magnetic flux. Also, the effect of being able to correct both the DC magnetization due to the DC component included in the output of self-commutated converter 3 and the DC magnetization due to the transient DC component included in power system 1 can be obtained. The following is a description of a further embodiment of this invention with reference to the schematic diagram in Figure 5. In Figure 5, 16 is a magnetic detector, such as a Hall element, for detecting the magnetic flux of the core of transformer 2. In Figure 5, the output voltage instruction of control circuit 7 is corrected using the value of magnetic flux of the core of transformer 2, instead of the current difference between the transformer primary winding current and the transformer secondary winding current in Figure 3. The following control is executed the same as in the embodiment shown in Figure 3. As shown in Figure 6, the relationship between excitation current and magnetic flux in transformer 2 is non-linear. The excitation current only gradually increases until the core of transformer 2 reaches saturation. However, when the core reaches saturation, it increases rapidly. Therefore, in order to suppress DC magnetization by correcting before saturation is reached, the detection of a DC component of a small excitation current, or a small variation in excitation current. In order to detect such a small variation, the detection of the excitation current must be highly accurate. However, usually, the magnitude of the excitation current is less than 10% of that of the rated current of a transformer. Therefore, when detecting the excitation current by the difference between the primary winding current and the secondary winding current, a current detector with an accuracy of 0.1% of the rated current is required in the case of detecting with an accuracy of 1% an excitation current the magnitude of which is 10% that of the rated current. When correcting the output voltage instruction value by detecting the magnetic flux of the transformer core as shown in Figure 5, more effective control can be exercised than the case when the correction is carried out by detecting the excitation current. Also, when using this embodiment, both the DC magnetization due to the DC component included in the output of self-commutated converter 3 and the DC magnetization due to the transient DC component included in power system 1 can be effectively corrected. In the embodiments shown in Figures 3 and 5, maximum value detection circuit 13 and minimum value detection circuit 14 detect the maximum and the minimum values from the inputted values at every fundamental period of power system 1. This invention is not limited to these embodiments. The maximum and the minimum values may be detected from the inputted values once for a plurality of fundamental periods of power system 1. Furthermore, a center value is computed in arithmetic circuit 15, and correction of the instruction value of control circuit 7 is executed by the center value. Instead of the strict center value, an appropriate value representing magnetic flux may be calculated from the magnetic flux or the exciting current in transformer 2 by a different method, and this value then may then be used for the correction of the instruction value from the control circuit, if DC magnetization of transformer 2 can be judged based on such a magnetic flux relating value. In a power converter composed of self-turn-off devices which is connected to an outer system, such as a power system or a load, via a transformer, according to this invention, both the DC magnetization due to the DC component included in the power converter and the DC magnetization due to the transient DC component included in the outer system can be corrected by finding a value representing the magnetic flux of the core of the transformer, such as the difference between the DC primary winding current and the DC secondary winding current of the transformer, or a magnetic flux itself, and correcting the output voltage instruction value of the power converter based on the value representing magnetic flux.

Claims: A control system for a power converter (3) having a plurality of switching devices and connected to an outer system (1) through a transformer (2) provided with a core, the control system comprising: output voltage instruction value generating means (7) for generating an instruction value for an output voltage of said power converter (3); correction means (5, 8, 9) for correcting said instruction value to generate a corrected instruction value for said output voltage of said power converter (3); and signal generating means (10, 11) for generating a signal based on said corrected instruction value, said signal being applied to said switching devices of said power converter (3) to control said output voltage of said power converter (3), characterised in that: said correction means (5A, 5B, 8, 9, 12; 5A, 5B, 9, 12, 13, 14, 15; 9, 13, 14, 15, 16) generates a corrected instruction value in dependence upon a value representing the DC component of the magnetic flux of the core of the transformer (2). A control system as claimed in claim 1, wherein said correction means includes: magnetic flux detecting means (16) for detecting a magnetic flux of said core of said transformer (2); maximum value detecting means (13) connected to receive said magnetic flux for detecting a maximum value of said magnetic flux during a predetermined time: minimum value detecting means (14) connected to receive said magnetic flux for detecting a minimum value of said magnetic flux during said predetermined time; centre value calculating means (15) for calculating a centre value from said maximum value and said minimum value during said predetermined time, said centre value being said value representing the DC component of the magnetic flux; and corrected instruction value generating means (9) connected to receive said instruction value and said value representing the DC component of the magnetic flux for generating said corrected instruction value for said output voltage of said power converter (3). The control system for a power converter according to claim 2: wherein said correction means further includes a phase detector (19) for detecting a fundamental period of said outer system (1); wherein in said correction means; said magnetic flux detecting means (16) includes a Hall element provided in said transformer (2) for detecting said magnetic flux of said core of said transformer (2); said maximum value detecting means (13) includes a maximum value detection circuit connected to receive said magnetic flux and said fundamental period as said predetermined time for detecting said maximum value of said magnetic flux during said fundamental period; said minimum value detecting means (14) includes a minimum value detection circuit connected to receive said magnetic flux and said fundamental period as said predetermined time for detecting said minimum value of said magnetic flux during said fundamental period; said centre value calculating means (15) includes an arithmetic circuit for calculating said centre value from said maximum value and said minimum value during said fundamental period, said centre value being said value representing the DC component of the magnetic flux; and said correction instruction value generating means (9) includes an adder for adding said instruction value and said value representing the DC component of the magnetic flux to generate said corrected instruction value. A control system as claimed in claim 1 wherein said correction means includes: current difference detecting means (5A, 5B, 12) for detecting a current difference between a primary winding current (i 1 ) and a secondary winding current (i 2 ) of said transformer (2); maximum value detecting means (13) connected to receive said current difference for detecting a maximum value of said current difference during a predetermined time; minimum value detecting means (14) connected to receive said current difference for detecting a minimum value of said current difference during said predetermined time; centre value calculating means (15) for calculating a centre value from said maximum value and said minimum value during said predetermined time, said centre value being said value representing the DC component of the magnetic flux; and corrected instruction value generating means (9) connected to receive said instruction value and said value representing the DC component of the magnetic flux for generating said corrected instruction value for said output voltage of said power converter. The control system for a power converter according to claim 4: wherein said correction means further includes a phase detector (19) for detecting a fundamental period of said outer system (1); wherein in said correction means; said current difference detecting means includes; a primary current detector (5B) for detecting said primary winding current of said transformer; a secondary current detector (5A) for detecting said secondary winding current of said transformer, a subtractor (12) for computing said current difference between said primary winding current and said secondary winding current; said maximum value detecting means (13) includes a maximum value detection circuit connected to receive said current difference and said fundamental period as said predetermined time for detecting said maximum value of said current difference during said fundamental period; said minimum value detecting means (14) includes a minimum value detection circuit connected to receive said current difference and said fundamental period as said predetermined time for detecting said minimum value of said current difference during said fundamental period; said centre value calculating means (15) includes an arithmetic circuit for calculating said centre value from said maximum value and said minimum value during said fundamental period, said centre value being said value representing the DC component of the magnetic flux, and said corrected instruction value generating means includes an adder (9) for adding said instruction value and said value representing the DC component of the magnetic flux to generate said corrected instruction value. A control system as claimed in claim 1 wherein said correction means includes: current difference detecting means (5A, 5B) for detecting a current difference between a primary winding current and a secondary winding current of said transformer; DC component detecting means (8) for detecting the DC component included in said current difference, said DC component being said value representing the DC component of the magnetic flux; and corrected instruction value generating means (9) connected to receive said instruction value and said value representing the DC component of the magnetic flux for generating said corrected instruction value for said output voltage of said power converter. The control system for a power converter according to claim 6, wherein said current difference detecting means includes: a primary current detector (5B) for detecting said primary winding current of said transformer; a secondary current detector (5A) for detecting said secondary winding current of said transformer; and a subtractor (12) for computing said current difference between said primary winding current and said secondary winding current. The control system for a power converter according to claim 6: wherein the winding ratio of said transformer is 1 : n; wherein said current difference detecting means includes; a primary current detector (5B) for detecting said primary winding current of said transformer; a secondary current detector (5A) for detecting said secondary winding current of said transformer; a multiplier (17) for multiplying said secondary winding current by n to generate a multiplied secondary winding current; and a subtractor (12) for computing a difference between said primary winding current and said multiplied secondary winding current to output said difference as said current difference. The control system for a power converter according to claim 6: wherein said power converter (3) is a three-phase power converter, and said transformer (2) is composed of a star-delta connection; wherein said current difference detecting means includes; primary current detecting means (5B1, 5B2, 5B3) for detecting said primary winding current of said transformer; secondary current detecting means (5A1, 5A2, 5A3) for detecting said secondary winding current of said transformer; star-delta conversion means (18) for converting said primary winding current into converted primary winding current; and subtractor means (121, 122, 123) for computing a difference between said converted primary winding current and said secondary winding current, said difference being said current difference. The control system for a power converter according to any preceding claim, wherein said outer system includes one of a power system and a load.

Inventors: KAWAKAMI NORIKO, KAWAKAMI, NORIKO, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1994
Main classification: H02M7
IPC codes: H02M7, H02J3, G05F1